Strain gages are utilized to sense and evaluate small linear deformation, and in the usual application the strain gage is firmly affixed to the deforming member such that lengthening of the member due to external forces likewise lengthens the strain gage producing an electric signal that may be diagnosed in terms of distance. Strain gages are often employed in the design of mechanical devices subjected to tension or compression forces and are utilized to analyze strength characteristics and deformation under known loads.
Strain gages have been used with metal processing equipment, such as rolling mills and machine tools, wherein high accuracies are required but due to high pressure forces being imposed upon the apparatus deformation in the metal processing occurs which will result in inaccuracies in the finished product unless compensation occurs. Metal processing equipment is formed of metal which expands and contracts due to thermal influence, and the accuracy of metal processing equipment will depend upon thermal influences as well as external forces being applied thereto. Accordingly, when processing metal under close dimensional tolerances both the metal deformation due to mechanical forces and thermal influences must be considered.
For instance, in metal rolling installations opposed rollers mounted within bearing blocks are employed to impose high compressive forces upon hot steel. The combination of the forces imposed upon the bearing block due to the rolling pressures required, and the high temperature environment, causes the bearing blocks to elongate and expand as the temperature thereof rises during operation, and such temperature changes will vary spacing between the rollers, and the thickness of the steel being rolled, unless adjustment of the roller spacing occurs. Accordingly, there is a great need in the metal rolling arts for means to determine dimensional variations wherein bearing block dimensional changes directly represent mechanical force characteristics and are free of "false readings" due to thermal conditions.
Electrical gaging devices which are thermally compensated are known, and have been used in machine tool and similar applications; typical devices of this type being shown in U.S. Pat. Nos. 2,664,787; 3,045,510; 3,332,153 and 3,775,655. However, such thermal compensation as shown in these patents does not compensate for thermal influences of the gaging equipment itself, and inaccuracies occur due to thermal influences on the gaging apparatus and thermal compensation becomes a compromise between several factors. U.S. Pat. Nos. 3,236,124 and 3,921,300 also disclose temperature compensating systems, and in the latter patent conditions within the gage, itself, are sensed, but patents of this type still fall short of producing a high degree of accuracy when measuring small dimensional variations.
It is an object of the invention to provide a thermally compensated gage capable of electrically indicating mechanical deformation within a metal member, the dimensional signal being substantially free of thermal influence.
A further object of the invention is to provide a thermally compensated gage producing an electrical signal proportional to mechanical deformation substantially free of thermal influence wherein very small mechanical dimensions can be sensed, and yet the apparatus is of a concise and relatively economical character.
Another object of the invention is to provide a thermally compensated gage producing an electrical signal indicating dimensional variation wherein the structure producing thermal compensation is economically combined with the structure indicating mechanical deformation to reduce the complexity of gage construction and minimize cost.
Yet a further object of the invention is to provide a thermally compensated gage utilizing a pair of linear variable differential transformers wherein a substantially uniform temperature is maintained within the gage to assure accurate compensation regardless of conditions.
In the practice of the invention an elongated aluminum rod is affixed substantially at its central region to the metal member to be measured. A component of a linear variable differential transformer, such as the core, is attached to this rod at a location remote from the anchor point. The coil of the LVDT is also affixed to the element to be measured in axial alignment with the aluminum rod, and in this manner mechanical deviations in the metal member between the attachment points of the aluminum rod and LVDT are sensed to produce an electrical signal transmitted to an analyzing and amplifying circuit.
The temperature compensating structure includes a stainless steel rod disposed parallel to the aluminum rod which is attached to the end region of the aluminum rod remote from the previously mentioned LVDT component. The other end of the stainless steel rod includes a component, such as a core, of a second LVDT. The coil of the second LVDT is attached to the aluminum rod adjacent the first LVDT component, and thus, an electrical signal is produced by the second LVDT proportional to changes occurring within the aluminum and stainless steel rod due to thermal influence. The second LVDT is also connected to the analyzing and amplification circuit. The LVDT associated with the aluminum rod provides a signal which varies directly with changes in temperature and with the application of force upon the element being measured, while the LVDT associated with the stainless steel rod provides a signal which varies inversely with changes in temperature. These two signals are summed such that the level of the summed signals does not vary with changes in temperature, and accordingly, the level of the summed signals varies only in readout, as the application of force and mechanical deformation occurs.
In order that the aforementioned apparatus be influenced by actual temperature conditions the rods and LVDTs are encased within a housing attached to the member being sensed, and in this manner the rods will assume a temperature proportional to the temperature of the element being measured, and will not be influenced by radiant heat or the cooling movement of air not directly associated with the element being measured.